Closure approximation in the theory of two-photon double ionization of atoms

We consider two-photon double ionization of helium with 100, 200, and 400 eV excess energy. We focus on the case of ultrashort pulses and develop an approach to calculate the two-photon transition matrix elements within the lowest order of the time-dependent perturbation theory. It is shown, that in the subfemtosecond regime the intermediate Green's function can be successfully evaluated by means of the closure approximation. © Published under licence by IOP Publishing Ltd